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DSLX Language Server

Many popular editors in the modern era are speaking a common protocol in order to understand how to display, traverse, and maniulate languages: the "Language Server Protocol". This allows novel languages and DSLs, like DSLX, to expose a developer experience integrated in their preferred editors and IDEs.

To use the language server protocol in your editor you do not need to know any details about the language server protocol.

Language server feedback in the editor is useful to folks who are learning DSLX as well as those developing in it on a regular basis! The language server currently offers functionality such as:

  • Go-to-definition
  • Errors/warnings as you type
  • An "overview" of the symbols defined in a module

For more background on what the language server protocol can do, see the Language Server Protocol documentation and specification.

Building the DSLX Language Server binary

The following are instructions for building the DSLX language server binary. By a) placing the language server binary into your PATH and b) configuring your editor to handle .x files by using it.

Follow the XLS build setup instructions so that the pre-requisites are available for building binaries via Bazel. Then, build the following dslx_ls binary and place it in your PATH:

~/xls$ bazel build -c opt //xls/dslx/lsp:dslx_ls
~/xls$ mkdir ~/bin/
~/xls$ cp -iv bazel-bin/xls/dslx/lsp/dslx_ls ~/bin/
~/xls$ export PATH=$HOME/bin:$PATH

Now that the language server binary is available in your PATH, you must configure your editor to find/use it for .x files.


First we must install vim-plug -- follow the latest instructions at; e.g.:

$ curl -fLo ~/.vim/autoload/plug.vim --create-dirs \

Add the following configuration to your $HOME/.vimrc:

call plug#begin()
Plug 'prabirshrestha/vim-lsp'
Plug 'prabirshrestha/asyncomplete-lsp.vim'
Plug 'mattn/vim-lsp-settings'

call plug#end()

let g:lsp_log_verbose = 1
let g:lsp_log_file = expand('~/vim-lsp.log')

if executable('dslx_ls')
    au User lsp_setup call lsp#register_server({
        \ 'name': 'dslx_ls',
        \ 'cmd': {server_info->['dslx_ls']},
        \ 'allowlist': ['dslx', '.x'],
        \ })

let g:lsp_diagnostics_echo_cursor = 1
let g:lsp_diagnostics_highlights_enabled = 1
let g:lsp_diagnostics_signs_enabled = 1

au BufRead,BufNewFile *.x set filetype=dslx

Run vim and execute :PlugInstall to get the new LSP plugins. After that completes successfully, quit Vim (famously, via :q).

Open foo.x in vim and then paste the following contents:

fn main() -> u8 { u8:256 }

The following should show in the display line:

LSP: uN[8] Value '256' does not fit in the bitwidth of a uN[8] (8). Valid values are [0, 255].

If not, try using :LspStatus To see if any diagnostics are available.

If you correct the u8 value to be 255 (and thus in range) you can run:


To see the defined symbol listing -- this shows all defined symbols in the file:

foo.x|1 col 1| method : main


With the above .vimrc contents, logs should show up in $HOME/vim-lsp.log.

Issues can be filed against with associated contents/logs.


The following .emacsrc snippet wires up the language server in emacs, piggy-backing on the Rust major mode.

(require 'lsp-mode)
(add-to-list 'auto-mode-alist '("\\.x\\'" . rust-mode))
;; DSLX can make rust-mode very slow for large files due to angle bracket
;; matching which is inefficiently implemented. Disable the feature.
(setq rust-match-angle-brackets nil)
(add-to-list 'lsp-language-id-configuration '(rust-mode . "dslx"))
 (make-lsp-client :new-connection (lsp-stdio-connection "~/bin/dslx_ls")
                  :major-modes '(rust-mode)
                  :server-id 'dslx-ls))
(add-hook 'rust-mode-hook 'lsp)

Additional details on Emacs enablement may be available in the Verible editor hook-up documentation.

Sublime Text

Create a DSLX syntax

Go to Tools > Developer > New Syntax, then replace values in the template:

  • Change the file_extensions value to be x
  • Change scope to be source.dslx
  • Replace all example-c in the file with dslx

Now when you open a .x file it should show the syntax in the lower right-hand corner as dslx.

Install LSP package

Instructions for installing the LSP package are given in the Verible documentation.

Add DSLX to LSP settings

Open the configuration via Preferences > Package Settings > LSP > Settings and add the following client:

    "clients": {
        "dslx_ls": {
          "command": ["dslx_ls"],
          "enabled": true,
          "selector": "source.dslx"

Now open foo.x and paste in:

fn main() -> u8 { u8:256 }

There should be a red squiggle under the number 256 indicating that the value is out of range for a u8.

Other Editors

For non-Vim editors, see the instructions provided by our sister project, Verible.